Analog and digital electronic circuitry and attendant wiring may encounter serious operating difficulty in the presence of strong electromagnetic radiation fields. Such radiation fields are generally referred to as Electromagnetic Interference (EMI) fields. The circuits and attendant wiring may be shielded and filtered to provide some immunity to large EMI fields. Methods and apparatus, therefore, are required to test the susceptibility of the circuits and attendant wiring to EMI fields.
EMI testing is typically performed in shielded enclosures known as "screen rooms" or faraday cages, which provide an electromagnetic environment wherein only controlled EMI fields are present. Controlled EMI fields include, but are not limited to radiated near and far fields, stripline and TEM testing in the range of DC (more typically 10 KHz) to 18 GHz.
Apparatus typically used inside the screen room includes current probes attached to a harness wire and a coaxial cable which sends the signals detected by the probes to a receiver outside the screen room, where the effects of the EMI fields on the circuit are determined. Current probes suitable for monitoring current during EMI tests are commercially available. The Ailtech model number 91197-11 is one such device. Current probes, however, are not able to measure signals in the device under test in many circumstances, for example, at trace conductors of integrated circuits or into open circuits. For such signals, voltage probes are better suited.
To ensure the integrity of the screen room and the results of the EMI tests, any voltage measuring apparatus within the screen room should minimally perturb the controlled EMI fields and should be energized by a signal from the device under test only. For example, any test apparatus which might reradiate EMI fields impinging on the device under test or might otherwise inject any noise into the device under test must be avoided.
U.S. Pat. 4,939,446, which is assigned to the assignee of this invention, refers to one such voltage probe transmission link that is transparent to electromagnetic radiation fields for use in screen room testing. The transmission link uses a voltage probe, which includes a circuit grabber, such as a short insulated conducting clip, which is connected to an electrically overdamped input conductor. The circuit grabber is connected to the test point of the device under test. The insulation on the clip surface is coated, with any bright, metallic reflecting material, such as a silver paint or foil, to shield the clip from impinging EMI fields, thereby preventing the injection of signals into the device under test by the clip. The other end of the input conductor is connected to a hybrid electrical/optical data transmitter having a high impedance input port, which also is located inside the screen room. The transmitter has an optical output port that is connected to a receiver by way of an EMI immune optical fiber. The receiver is located outside the screen room where the effects of controlled EMI fields on the device under test are monitored, outside of the test electromagnetic radiation field.
The voltage probe input conductors may comprise a non-metallic thread core that is impregnated with fine conducting particles and a rigid, non-metallic insulating sheath. The electrically overdamped input conductors have a high distributed resistance so that they will not ring or tune at the frequencies of interest and, therefore, will not pick up energy from the EMI fields. As a result, the voltage probe transmission link may be used to monitor voltages of a device under test in the presence of a strong EMI field without effecting the device under test or the test results. The disclosure of U.S. Pat. No. 4,939,446 is hereby incorporated in its entirety herein by reference.
A commercial product, known by the tradename ETVL (Electromagnetically Transparent Voltage Monitor Link System), available from the assignee of this invention, Electronic Development Inc., is a commercial version of the voltage probe transmission link described in U.S. Pat. No. 4,934,446. The ETVL product has a hybrid electrical/optical data transmitter that has a single transmission channel that may have one of three signal formats, namely analog, digital, and pulse stretched. Only one signal format can be used at a time on the one transmission channel.
The commercial ETVL device and the device described in U.S. Pat. No. 4,939,446 monitor only a single test point and one voltage waveform (single ended or double ended) of the device under test. They also use one voltage probe for providing a return current path from the transmitter to the device under test.